1. Field of the Invention
Since a micro electro mechanical system (MEMS) is a functional element having a movable part, the MEMS needs a cavity as an operation space for the movable part, and the cavity is airtightly sealed so as to prevent the intrusion of outside air and protect the functional element.
2. Description of the Related Art
A functional element which is airtightly sealed in a cavity formed by etching a sacrificial film on a substrate has been known. As regards the functional element disclosed in JP 2006-7459, a functional element on a silicon substrate is covered with a sacrificial film, and an etching-resistant film having an opening on the sacrificial film is formed. Etching the sacrificial film through the opening forms the cavity by means of the sacrificial film, and the functional element is stored in the cavity. After this, since a silicon nitride film is formed on the sacrificial film, the opening is sealed, and then the inside of the cavity is airtightly sealed.
However, in the technique disclosed in JP 2006-7459, since the silicon nitride film has strong compression force to a Si substrate, an etching-resistant film forming a cavity is deformed, and the cavity deforms with time.
The stress of the film is lowered by thinning a silicon nitride film to be used in the sealing the opening. However, in a case where the silicon nitride film is thinned, it is needed to make the size of the opening sufficiently small so as to prevent the silicon nitride film dropping into the cavity from the opening and to seal the opening. Therefore, the technique has a problem that it takes a long time to remove the sacrificial film from the small opening and that the sacrificial film remains in the cavity due to a shortage of etching. Thus, an electrical device with a higher reliability has been desired.